CN113381003B - Method for modifying lithium metal surface by mixed gas in grading manner and lithium metal battery - Google Patents
Method for modifying lithium metal surface by mixed gas in grading manner and lithium metal battery Download PDFInfo
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Abstract
The invention relates to the field of lithium batteries, and discloses a method for modifying a lithium metal surface by mixed gas in a grading manner and a lithium metal battery. By controlling the proportion of the mixed gas, lithium sulfate and a small amount of lithium fluoride crystal grains are formed on the surface of the lithium metal. And then, by changing the proportion of the mixed gas, more lithium fluoride is formed on the inner layer of the lithium metal surface and the lithium sulfate surface. The lithium fluoride of the inner layer and the lithium fluoride of the middle layer are conducted, and the conductivity of the lithium metal interface is improved. The lithium sulfate in the middle layer and the lithium sulfate on the surface are less, so that the reaction of lithium metal with water and oxygen in the air is inhibited, and the requirements on storage and use environments are reduced. Meanwhile, the capacity retention rate and the cycle performance of the lithium metal battery assembled by the lithium metal treated by the process are obviously improved.
Description
Technical Field
The invention relates to the field of lithium batteries, in particular to a lithium metal battery and a lithium metal battery surface modification process thereof.
Background
Lithium metal is a silvery-white, soft metal, and is also the least dense metal. Lithium metal has an extremely low potential for hydrogen, is an ideal negative electrode material in lithium batteries, and is expected to be widely applied to future lithium metal batteries. However, lithium metal stored in air reacts slowly with water, oxygen, nitrogen, etc. in the air, and loses its surface properties after being corroded. Therefore, lithium metal is generally stored in a glove box, which makes storage of lithium metal extremely inconvenient.
The existing method for processing the lithium metal surface by single gas is too single or the reaction is too violent, and the generated surface protective film has defects in performance and is difficult to ensure the coexistence of plasticity, conductivity and stability. For example, in the method of treating the surface of lithium metal with carbon dioxide, the stability of the produced lithium carbonate is poor; the reaction that occurs when lithium fluoride is used alone to treat the lithium metal surface is too vigorous to control. Therefore, it is an urgent need to provide a method for modifying a lithium metal surface to form a protective film with good stability, without affecting conductivity and easy to control.
Disclosure of Invention
The invention aims to solve the technical problem of providing a method for modifying the surface of lithium metal by mixed gas in a grading way and a lithium metal battery, so that a protective film with a composite structure containing lithium sulfate and lithium fluoride is generated on the surface of the lithium metal, the direct contact of the lithium metal and air can be avoided, the conductivity of the lithium metal is not influenced, great convenience is brought to the storage of the lithium metal, the surface performance of the lithium metal is improved, and the method has the advantages of simple process, easiness in operation, good repeatability and easiness in realization of large-scale industrial production.
The invention provides a lithium metal surface modification process of a lithium metal battery and the lithium metal battery.
In a first aspect, the present invention provides a lithium metal surface modification process for a lithium metal battery, which adopts the following technical scheme:
a method for mixed gas graded modification of a lithium metal surface, the method comprising the steps of:
s1: firstly, putting lithium metal into a closed container, vacuumizing the closed container, and introducing a first mixed gas of protective gas, hydrogen fluoride, sulfur dioxide and oxygen at a certain pressure;
s2: reacting lithium metal and a first mixed gas of protective gas, hydrogen fluoride, sulfur dioxide and oxygen in a closed container at a certain temperature for a period of time, and then evacuating the mixed gas to obtain primary surface modified lithium metal;
s3: vacuumizing the closed container, and introducing a protective gas with a certain pressure, a second mixed gas of hydrogen fluoride and sulfur dioxide, wherein the ratio of hydrogen fluoride in the second mixed gas is larger than that of hydrogen fluoride in the first mixed gas in the step S1;
s4: and (3) reacting the primary surface modified lithium metal in the closed container with a second mixed gas of protective gas, hydrogen fluoride and sulfur dioxide at a certain temperature for a period of time, and then emptying the mixed gas to obtain the final surface modified lithium metal.
Preferably, the lithium metal is in the form of a strip, a block or a powder, and the protective gas in the first mixed gas and the second mixed gas is at least one of argon, helium and neon.
Preferably, in step S1, the first mixed gas is introduced at a pressure ranging from 1MPa to 5 MPa.
By adopting the technical scheme, the pressure of 1-5 MPa is adopted, so that the mixed gas can be promoted to react with the surface of lithium metal, and more compact lithium sulfate and less lithium fluoride crystal nucleus film layers are generated. Too much pressure will affect the thickness of the lithium sulfate film or break the film to fall off, and lead to the generation of a large amount of lithium fluoride, which is not favorable for forming a stable composite structure.
Preferably, in the step S3, the pressure of the second mixed gas is 6MPa to 10 MPa.
By adopting the technical scheme, the generation of lithium fluoride and the tight combination of lithium sulfate and lithium fluoride can be promoted by adopting the pressure of 6-10 MPa, and meanwhile, the conductivity of the SEI film is improved by the compact lithium fluoride. Too high a pressure may cause the film to break and fall off.
Preferably, in the step S2, the reaction temperature is 30 to 80 ℃.
Preferably, in the step S4, the reaction temperature is 30 to 50 ℃.
Preferably, in the step S2, the reaction time is 0.1 to 10 hours.
Preferably, in the step S4, the reaction time is 0.1 to 1 hour.
By adopting the technical scheme and the temperature and time in the steps, the generation rates of the lithium sulfate and the lithium fluoride can be milder, so that the required protective film layer with a composite structure can be formed. Higher temperature makes the reaction more violent, and too long reaction time affects the thickness of SEI and the ratio of lithium sulfide and lithium fluoride, and the desired effect cannot be achieved.
Preferably, in step S1, the ratio of hydrogen fluoride in the first mixed gas is in a range of 1% to 5%, the ratio of sulfur dioxide is in a range of 50% to 70%, and the ratio of oxygen is in a range of 10% to 20%.
Preferably, in step S3, the ratio of hydrogen fluoride in the second mixed gas is in a range of 5% to 10%, and the ratio of sulfur dioxide is in a range of 30% to 50%.
By adopting the technical scheme, more lithium sulfate and less lithium fluoride crystal grains can be formed in the step S2 by adopting the gas proportion in the step S3, the proportion of the hydrogen fluoride is increased, more lithium fluoride is formed on the inner layer of the lithium metal surface and the lithium sulfate surface, and the lithium sulfate and the lithium fluoride have good bonding performance by utilizing the in-situ reaction of the lithium sulfate and the hydrogen fluoride. On the basis of the gas ratio in the above steps, decreasing the ratio of sulfur dioxide or increasing the ratio of hydrogen fluoride affects the thickness ratio of SEI or forms a large amount of lithium fluoride, which is not favorable for forming a stable composite structure.
In a second aspect, the present invention provides a lithium metal battery, which adopts the following technical scheme:
a lithium metal battery is assembled by adopting the lithium metal obtained by the method for modifying the surface of the lithium metal by mixed gas in a grading way.
By adopting the technical scheme, the SEI film formed by the lithium metal cathode of the lithium metal battery obtained by adopting the surface modification process is stable and compact, so that the lithium metal battery has longer service life and higher cycle performance.
Compared with the prior art, the invention has the following beneficial effects,
1. the method for modifying the surface of the lithium metal by mixed gas in a grading way can avoid the direct contact of the lithium metal and air, prevent the lithium metal from slowly reacting with water, oxygen, nitrogen and the like in the air and reduce the storage requirement of the lithium metal;
2. and forming a composite structure SEI film of lithium fluoride and lithium sulfate with sufficient thickness and stable structure on the surface of the lithium metal by a method for modifying the surface of the lithium metal by mixing gases in different proportions in a grading manner. The lithium sulfate and lithium metal bond more strongly, acting as a rivet in this composite structure, can better fix the SEI structure and is therefore more stable. In addition, the lithium fluoride conducting between the inner layer and the outer layer enables the surface of the lithium metal to have good conductivity, and improves the surface stability of the lithium metal;
3. the lithium metal obtained by the method for modifying the surface of the lithium metal by mixed gas grading with simple process has a stable, uniform and compact SEI film, and the lithium metal battery assembled by the lithium metal obviously improves the capacity retention rate and the cycle performance of the battery.
Drawings
Fig. 1 is a schematic diagram illustrating a structural change of the lithium metal surface in this embodiment.
Detailed Description
The present invention is described in further detail below with reference to examples.
A method for modifying the surface of lithium metal by mixed gas classification comprises the following steps:
s1: firstly, putting lithium metal into a closed container, vacuumizing the closed container, and introducing a first mixed gas of protective gas, hydrogen fluoride, sulfur dioxide and oxygen at a certain pressure;
s2: reacting a first mixed gas of lithium metal and protective gas, hydrogen fluoride, sulfur dioxide and oxygen in a closed container at a certain temperature for a period of time, and then evacuating the mixed gas to form lithium sulfate and less lithium fluoride crystal grains on the surface of the lithium metal, thereby obtaining primary surface modified lithium metal;
s3: vacuumizing the closed container, and introducing a second mixed gas of protective gas, hydrogen fluoride and sulfur dioxide at a certain pressure, wherein the proportion of the hydrogen fluoride in the second mixed gas is larger than that of the hydrogen fluoride in the first mixed gas in the step S1;
s4: and (3) reacting the primary surface modified lithium metal in the closed container with a second mixed gas of protective gas, hydrogen fluoride and sulfur dioxide at a certain temperature for a period of time, and then evacuating the mixed gas to form more lithium fluoride on the inner layer of the lithium metal surface and the lithium sulfate surface, thus obtaining the final surface modified lithium metal.
Specifically, the lithium metal is in the form of a strip, a block or a powder, and the protective gas in the first mixed gas and the second mixed gas is at least one of argon, helium and neon respectively.
In step S1, the pressure range of the first mixed gas is 1MPa to 5 MPa. The pressure of 1 MPa-5 MPa is adopted, so that the mixed gas can be promoted to react with the surface of lithium metal, and a compact composite structure film layer of lithium sulfate and less lithium fluoride crystal grains is generated. Too much pressure will affect the thickness of the lithium sulfate film or break the film to fall off, and lead to the generation of a large amount of lithium fluoride, which is not favorable for forming a stable composite structure.
In step S1, the first mixed gas contains hydrogen fluoride in a proportion range of 1% to 5%, sulfur dioxide in a proportion range of 50% to 70%, and oxygen in a proportion range of 10% to 20%. With this gas ratio, more lithium sulfate and less lithium fluoride crystal grains can be formed in step S2. Too large a proportion of hydrogen fluoride results in the formation of a large amount of lithium fluoride, which is not favorable for the formation of a stable composite structure.
In step S3, the pressure range of the second mixed gas is 6MPa to 10 MPa. The pressure of 6 MPa-10 MPa is adopted, so that the generation of lithium fluoride and the tight combination of lithium sulfate and lithium fluoride can be promoted, and meanwhile, the conductivity of the SEI film is improved by the compact lithium fluoride. Too high a pressure may cause the film to break and fall off.
In step S3, the hydrogen fluoride in the second mixed gas has a ratio in the range of 5% to 10%, and the sulfur dioxide has a ratio in the range of 30% to 50%. In step S3, the ratio of hydrogen fluoride is increased to form more lithium fluoride on the inner layer of the lithium metal surface and the lithium sulfate surface, and the lithium sulfate and the lithium fluoride have good bondability by the in-situ reaction of the lithium sulfate and the hydrogen fluoride. On the basis of the gas ratio in the above steps, decreasing the ratio of sulfur dioxide or increasing the ratio of hydrogen fluoride affects the thickness ratio of SEI or forms a large amount of lithium fluoride, which is not favorable for forming a stable composite structure.
In step S2, the reaction temperature is 30-80 ℃ and the reaction time is 0.1-10 h. In step S4, the reaction temperature is 30-50 ℃ and the reaction time is 0.1-1 h.
By adopting the temperature and the time, the generation rate of the lithium sulfate and the lithium fluoride can be milder, so as to form a required protective film layer with a composite structure. Higher temperature leads to more violent reaction, and longer reaction time affects the thickness of SEI and the proportion of lithium sulfide and lithium fluoride, which can not achieve the required effect.
A lithium metal battery is assembled by the lithium metal obtained by the method for modifying the surface of the lithium metal by mixed gas in a grading manner. The battery assembled by the lithium metal negative electrode obtained by the surface modification process has the advantages that an SEI film formed by the lithium metal negative electrode is stable and compact, so that the service life and the cycle performance are higher.
In the following examples and comparative examples, soft-packed lithium metal batteries were used, in which the positive electrode material was NCM523, the negative electrode material was lithium metal 50 μ M, and the electrolyte was 1M LiPF 6 The separator is conventional PP, and the battery capacity is 5 Ah. And the positive electrode and the negative electrode of the lithium metal battery are in a sheet shape.
Examples 1,
Firstly, processing a negative electrode material by the method for modifying the surface of the lithium metal in a grading manner by using the mixed gas, and then assembling a positive plate and the processed negative plate into a soft package lithium metal battery.
Selecting a plurality of anode materials with the same size as parallel samples, wherein the surface modification process of the anode materials comprises the following specific steps:
s1: firstly, putting lithium metal into a closed container, vacuumizing the closed container, and introducing a first mixed gas with the pressure of 3MPa, wherein the proportion of argon, sulfur dioxide, hydrogen fluoride and oxygen in the first mixed gas is 20%, 68%, 2% and 10%;
s2: keeping the temperature of a closed container filled with lithium metal and first mixed gas at 80 ℃ for reaction for 1 hour, and then evacuating the mixed gas to obtain primary surface modified lithium metal;
s3: vacuumizing the closed container, and introducing a second mixed gas with the pressure of 6MPa, wherein the proportions of argon, sulfur dioxide and hydrogen fluoride in the second mixed gas are 54%, 40% and 6%;
s4: and (3) carrying out heat preservation reaction on the container filled with the lithium metal and the second mixed gas at the temperature of 40 ℃ for 0.5h, and then emptying the mixed gas to obtain the final surface modified lithium metal.
Examples 2,
This example is different from example 1 in that the specific parameters for modifying the surface of the anode material are different, that is, in step S1, the proportions of argon, sulfur dioxide, hydrogen fluoride and oxygen in the first mixed gas are 13%, 70%, 2% and 15%.
Examples 3,
This example is different from example 1 in that the proportions of argon, sulfur dioxide, hydrogen fluoride and oxygen in the first mixed gas in step S1 were 39%, 50%, 1% and 10%.
Examples 4,
This example is different from example 1 in that the proportions of argon, sulfur dioxide, hydrogen fluoride and oxygen in the first mixed gas in step S1 were 28%, 50%, 2% and 20%.
Examples 5,
This example is different from example 1 in that the proportions of argon, sulfur dioxide, hydrogen fluoride and oxygen in the first mixed gas in step S1 were 25%, 60%, 5% and 10%.
Part of the parallel sample of the finally modified lithium metal obtained in examples 1 to 5 was exposed to air for 30min, and the color and size change of the surface was observed, so that it can be seen that the first mixed gas with the same pressure and different proportions was input into the closed container to react with the lithium metal, and the higher the proportion of sulfur dioxide and oxygen was under the same reaction conditions, the better the stability of the finally modified lithium metal was.
Examples 6,
This example is different from example 1 in that, in step S1, the pressure of the first mixed gas introduced into the closed vessel was 4MPa, and the proportions of argon, sulfur dioxide, hydrogen fluoride, and oxygen in the first mixed gas were 28%, 60%, 1%, and 11%.
Example 7,
This example is different from example 1 in that, in step S1, the pressure of the first mixed gas introduced into the closed vessel was 1MPa, and the proportions of argon, sulfur dioxide, hydrogen fluoride, and oxygen in the first mixed gas were 20%, 68%, 2%, and 10%.
Example 8,
This example is different from example 1 in that, in step S1, the pressure of the first mixed gas introduced into the closed container was 5MPa, and the proportions of argon, sulfur dioxide, hydrogen fluoride, and oxygen in the first mixed gas were 20%, 68%, 2%, and 10%.
Examples 9,
This example is different from example 1 in that, in step S1, the pressure of the first mixed gas introduced into the closed container was 1MPa, and the proportions of argon, sulfur dioxide, hydrogen fluoride, and oxygen in the first mixed gas were 28%, 50%, 2%, and 20%.
Examples 10,
This example is different from example 1 in that, in step S1, the pressure of the first mixed gas introduced into the closed container was 4MPa, and the proportions of argon, sulfur dioxide, hydrogen fluoride, and oxygen in the first mixed gas were 18%, 65%, 5%, and 12%.
The partial parallel samples of the final modified lithium metal obtained in example 1 and examples 6 to 10 were exposed to air for 30min, and the color and size change of the surface was observed, and it can be seen that the first mixed gas with the same ratio was input into the closed container to react with the lithium metal, and under the same reaction conditions, the higher the pressure, the better the stability of the final modified lithium metal, and similar results can be obtained by adjusting the pressure and the ratio of the first mixed gas.
Examples 11,
This example is different from example 1 in that the reaction temperature is 70 ℃ and the reaction time is 2 hours in step S2.
Examples 12,
This example differs from example 1 in that in step S2, the reaction temperature was 80 ℃ and the reaction time was 0.5 h.
Examples 13,
This example differs from example 1 in that in step S2, the reaction temperature was 70 ℃ and the reaction time was 4 hours.
Examples 14,
This example is different from example 4 in that the reaction temperature is 30 ℃ and the reaction time is 10 hours in step S2.
Examples 15,
This example differs from example 4 in that in step S2, the reaction temperature was 30 ℃ and the reaction time was 1 h.
The samples of the finally modified lithium metal obtained in examples 1 and 11 to 15, which are partially parallel to each other, are exposed in the air for 30min, and the color and size change of the surface is observed, so that it can be seen that the first mixed gas with the same proportion is input into the closed container to react with the lithium metal, and the higher the reaction temperature is, the better the gloss of the finally modified lithium metal obtained after the finally modified lithium metal is exposed in the air is after the reaction time is longer, i.e. the better the stability of the finally modified lithium metal is, but if the reaction temperature is too low, the reaction speed is slow, the production efficiency is affected, the formation of a composite structure is also affected, and if the reaction temperature is too high, the reaction is too severe, and the formation of the composite structure is also affected.
Examples 16,
This example is different from example 1 in that the pressure of the second mixed gas introduced into the closed vessel in step S3 was 7MPa, and the proportions of argon, sulfur dioxide and hydrogen fluoride in the mixed gas were 43%, 50% and 7%.
Examples 17,
This example is different from example 1 in that the pressure of the second mixed gas introduced into the closed vessel in step S3 was 6MPa, and the proportions of argon, sulfur dioxide and hydrogen fluoride in the mixed gas were 43%, 50% and 7%.
Examples 18,
This example is different from example 1 in that the pressure of the second mixed gas introduced into the closed vessel in step S3 was 10MPa, and the proportions of argon, sulfur dioxide and hydrogen fluoride in the mixed gas were 43%, 50% and 7%.
After the partial parallel samples of the modified lithium metals obtained in example 1 and examples 16 to 18 were exposed to the air for 30min, the electrical conductivity of the modified lithium metals was tested, and it could be obtained that the electrical conductivity of the final surface modified lithium metal was positively correlated to the pressure of the input second mixed gas.
Examples 19,
This example differs from example 1 in that in step S4, the reaction temperature was 50 ℃ and the reaction time was 1 hour.
Examples 20,
This example is different from example 1 in that the reaction temperature is 30 ℃ and the reaction time is 1 hour in step S4.
Examples 21,
This example differs from example 1 in that in step S4, the reaction temperature was 50 ℃ and the reaction time was 0.1 h.
Examples 22,
This example differs from example 1 in that in step S4, the reaction temperature was 30 ℃ and the reaction time was 0.1 h.
After the modified lithium metal samples obtained in example 1 and examples 19 to 22 were exposed to air for 30min, the conductivity of the modified lithium metal was measured, and the conductivity of the final surface-modified lithium metal was found to be positively correlated to the reaction temperature and the reaction time.
Comparative examples 1,
Selecting a plurality of anode materials with the same size as parallel samples, and carrying out modification treatment on the surfaces of the anode materials, wherein the method comprises the following specific steps:
s1: firstly, putting lithium metal into a closed container, vacuumizing the closed container, and introducing carbon dioxide gas with the pressure of 8 MPa;
s2: and (3) carrying out heat preservation reaction on the closed container filled with the lithium metal and the carbon dioxide at the temperature of 80 ℃ for 1h, and then emptying the carbon dioxide to obtain the surface modified lithium metal.
And (3) assembling the lithium metal battery by taking the modified lithium metal as a negative electrode.
Comparative examples 2,
The difference from comparative example 1 is that:
s1: firstly, putting lithium metal into a closed container, vacuumizing the container, and introducing carbon dioxide gas with the pressure of 6 MPa;
s2: and (3) carrying out heat preservation reaction on the container filled with the lithium metal and the carbon dioxide at 70 ℃ for 2h, and then emptying the carbon dioxide to obtain the surface modified lithium metal.
Comparative examples 3,
The untreated lithium metal sheet serves as a negative electrode, and is assembled into a lithium metal battery.
The above experiment was performed by selecting lithium metals having the same size, and then exposing the modified lithium metals obtained in examples 1 to 22 and comparative examples 1 to 3 to humid air for 30min, and observing the change in color and size of the surface, it was possible to obtain that the modified lithium metals obtained in examples 1 to 22 were all larger than the modified lithium metals obtained in comparative examples 1 to 3 after being exposed to air for 30min, and the surface still had metallic luster, while the modified lithium metals obtained in comparative examples 1 to 2 and the lithium metal sheet of comparative example 3 had darkened and dull surfaces. The improved stability of the modified lithium metals obtained in examples 1-22 is demonstrated.
Compared with the prior art, the method for modifying the surface of the lithium metal by mixed gas in a grading manner and the lithium metal battery have the advantages that the mixed gas of argon, hydrogen fluoride, sulfur dioxide and oxygen in different proportions is used for carrying out multiple reactions with the surface of the lithium metal, and a uniform and compact lithium fluoride and lithium sulfate coating layer is constructed on the surface of the lithium metal in situ. By controlling the proportion of the mixed gas, lithium sulfate and a small amount of lithium fluoride crystal grains are formed on the surface of the lithium metal in the first step. And in the second step, the ratio of the mixed gas is changed, so that more lithium fluoride is formed on the inner layer of the lithium metal surface and the lithium sulfate surface. The lithium fluoride of the inner layer and the lithium fluoride of the outer layer are conducted with the lithium fluoride of the middle layer, so that the conductivity of a lithium metal interface is greatly improved, and the lithium fluoride has strong plasticity and can be well adapted to the volume deformation of lithium metal. And the lithium sulfate in the middle layer and the lithium sulfate on the surface are less, so that the reaction of lithium metal with water and oxygen in the air is inhibited, and the requirements on storage and use environments are reduced. And the binding energy of the lithium sulfate and the lithium is greater than that of the lithium fluoride and the lithium, so that the lithium sulfate and the lithium are combined more tightly, the lithium sulfate plays a role of a rivet in the structure, the whole structure is more stable, and the lithium sulfate can better adapt to the volume change of a lithium cathode in the circulating process. The surface modification method improves the surface performance of lithium, reduces the storage requirement of lithium metal and is beneficial to industrial production. Meanwhile, the lithium metal battery assembled by the lithium metal treated by the process has better capacity retention rate and cycle performance.
Performance test of lithium Metal Battery
Test 1: battery cycle performance test
The test method comprises the following steps: by adopting the pretreatment processes in the above examples 1 to 22 and comparative examples 1 to 3, the lithium metal battery was subjected to 0.2C/0.2C charge-discharge cycles, and when the cycle was completed until the capacity retention rate was 80%, the cycle number of the battery was recorded, and the test results are shown in table 1.
TABLE 1 Battery cycling Performance test results
| Number of cycles/time | Number of cycles/time | ||
| Example 1 | 230 | Example 14 | 150 |
| Example 2 | 245 | Example 15 | 130 |
| Example 3 | 210 | Example 16 | 235 |
| Example 4 | 220 | Example 17 | 232 |
| Example 5 | 225 | Example 18 | 240 |
| Example 6 | 230 | Example 19 | 235 |
| Example 7 | 170 | Example 20 | 230 |
| Example 8 | 235 | Example 21 | 170 |
| Example 9 | 160 | Example 22 | 160 |
| Example 10 | 240 | Comparative example 1 | 100 |
| Example 11 | 220 | Comparative example 2 | 110 |
| Example 12 | 170 | Comparative example 3 | 20 |
| Example 13 | 250 |
And (3) analyzing test results:
(1) it can be seen from the combination of examples 1 to 22 and comparative examples 1 to 3 and the combination of table 1 that the cycle performance of the lithium metal battery can be significantly improved by modifying the surface of the lithium metal with the mixed gas, and the best effect cannot be achieved without any operation. The reason for this is that lithium metal forms a uniform and dense SEI film on a battery negative electrode through mixed gas treatment to suppress the formation of lithium dendrites, thereby improving the cycle life of the lithium metal battery; furthermore, by controlling the proportion of the mixed gas and the sequence of introducing the mixed gas, the SEI film can form a composite structure of lithium sulfate and lithium fluoride, so that the compact and stable conditions required by the excellent SEI film are met, the conductivity condition required by lithium ion transmission is also met, and the cycle performance of the lithium metal battery is favorably improved.
(2) By combining the examples 1, 2 to 5 and 11 to 15 and combining table 1, it can be seen that, in the temperature range, the surface modification is performed on the lithium metal by using a high ratio of sulfur dioxide to oxygen in step S1, and the cycle performance of the lithium metal battery is improved by using a higher temperature and a longer reaction time in step S2. The reason is that the high proportion of sulfur dioxide and oxygen or the high temperature and the long reaction time are both helpful for forming a thicker and more proportion of lithium sulfate SEI film, effectively preventing the formation of lithium dendrites, thereby improving the cycle performance of the lithium metal battery.
(3) It can be seen from the combination of the embodiment 1 and the embodiments 6 to 10 and the table 1 that, in the pressure range, the higher the pressure of the mixed gas used in the step S1 is, the more advantageous the improvement of the cycle performance of the lithium metal battery is. The reason for this is that the higher the pressure, the more dense and stable the formed lithium sulfate SEI film layer is, and the tighter the bonding with the lithium metal surface is, so that the cycle performance of the lithium metal battery is also better.
(4) With reference to examples 1, 16 to 18, and 19 to 22, and with reference to table 1, it can be seen that, in the above pressure, temperature, and time, the higher pressure used in step S3 and the higher reaction temperature or reaction time used in step S4 are both beneficial to improving the cycle performance of the lithium metal battery. The reason for this is that higher pressure increases the density of the lithium fluoride and higher reaction temperature or reaction time increases the thickness of the lithium fluoride. The lithium fluoride has excellent conductivity, and reduces the interface resistance of the lithium metal surface, thereby improving the cycle performance of the lithium metal battery.
Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that modifications and variations of the present invention are possible to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. A method for modifying the surface of lithium metal by mixed gas in a grading way is characterized in that: the method comprises the following steps:
s1: firstly, putting lithium metal into a closed container, vacuumizing the closed container, and introducing a first mixed gas of protective gas, hydrogen fluoride, sulfur dioxide and oxygen, wherein the pressure range of the first mixed gas is 1-5 MPa;
s2: reacting a first mixed gas of lithium metal and protective gas, hydrogen fluoride, sulfur dioxide and oxygen in a closed container at the temperature of 30-80 ℃ for a period of time, and then evacuating the mixed gas to obtain primary surface modified lithium metal;
s3: vacuumizing the closed container, and introducing a second mixed gas of protective gas, hydrogen fluoride and sulfur dioxide, wherein the pressure range of the second mixed gas is 6-10 MPa, and the proportion of the hydrogen fluoride in the second mixed gas is larger than that of the hydrogen fluoride in the first mixed gas in the step S1;
s4: and (3) reacting the primary surface modified lithium metal in the closed container with a second mixed gas of protective gas, hydrogen fluoride and sulfur dioxide at the temperature of between 30 and 50 ℃ for a period of time, and then evacuating the mixed gas to obtain the final surface modified lithium metal.
2. The method for modifying the surface of lithium metal in a mixed gas classification way according to claim 1, characterized in that: the lithium metal is strip, block or powder, and the protective gas in the first mixed gas and the second mixed gas is at least one of argon, helium and neon respectively.
3. The method for modifying the surface of lithium metal in a mixed gas classification way according to claim 1, characterized in that: in the step S2, the reaction time is 0.1-10 h.
4. The method for modifying the surface of lithium metal in a mixed gas classification way according to claim 1, characterized in that: in the step S4, the reaction time is 0.1-1 h.
5. The method for the mixed gas graded modification of the lithium metal surface according to claim 1, wherein the mixed gas is selected from the group consisting of: in step S1, the ratio of hydrogen fluoride in the first mixed gas is in the range of 1% to 5%, the ratio of sulfur dioxide is in the range of 50% to 70%, and the ratio of oxygen is in the range of 10% to 20%.
6. The method for modifying the surface of lithium metal in a mixed gas classification way according to claim 1, characterized in that: in step S3, the ratio of hydrogen fluoride in the second mixed gas ranges from 5% to 10%, and the ratio of sulfur dioxide ranges from 30% to 50%.
7. A lithium metal battery, characterized in that: the lithium metal assembly is obtained by assembling the lithium metal obtained by the method for modifying the surface of the lithium metal by the mixed gas classification according to any one of claims 1 to 6.
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